1. Field of the Invention
This invention relates to seals and, more specifically, to high-temperature and high-pressure seals for the pump industry.
2. Description of the Prior Art
The requirements for high-pressure centrifugal pumping in the chemical processes, mining, dredging, and solids transport industries at temperatures above about 500.degree. F. necessitate long life shaft seals that can operate reliably and efficiently under the most severe abrasive conditions with the pump.
Specifically, coal liquefaction processes presently being considered for commercial development will require a high-capacity and high-pressure coal slurry feed system in the process. A scale-up of present-type reciprocating feed pumps to meet the high capacity will require a large number of units with attendant high capital and maintenance costs. As an alternate approach, high-volume centrifugal pumps appear promising in meeting the high-capacity requirement of the coal liquefaction plant. However, centrifugal pumps must be designed for high speeds to meet the high-pressure requirements, and may be subject to excessive internal wear because of excessive slurry velocities.
The pump seal package for this kind of environment must be designed to be capable of sealing from 4 to 350-mesh (0.230 to 0.0018 inches) coal/oil slurry at a temperature of about 550.degree. F. and at pressures ranging from 250 psi for the first pump stage to above about 3000 psi for the final pressure stage. The seal package must operate reliably at pressure levels of 250 psi increments. The 3000 psi pressure level was used for the design, hydraulic, and dynamic analysis.
The use of a face-contacting, pressure-balanced seal as a primary barrier 3000 psi was found to be too sensitive to operate reliably over a long period primarily because of the high accuracy required for the seal pressure balance diameter that must be maintained for fine tune face pressure load in the presence of coal slurry. Other system conditions such as pump pressure oscillations or variations in the purge fluid pressure to the mechanical seal, thermal expansion of the various pump components, led to the conclusion that the seal cavity pressure must be reduced to a more controllable level for reliable mechanical seal operation.
Pressure reducing bleedback cavities which return controlled leaking slurry flow to a lower pump pressure level were analyzed and found that the leaking velocity was too high for long-period reliable operation and therefore caused extreme material wear.